Resonant grounding device for alternating-current circuits



Dec. 26, 1950 H. HARRISON 2,535,064

RESONANT GROUNDING DEVICE FOR ALTERNATING-CURRENT CIRCUITS Filed Feb. 24, 1948 INVENTOR Patented Dec. 26, 1950 UNITED STATES PATENT OFFICE RESONAN'I GROUNDING DEVICE. FOR ALTERNATING-CURRENT CIRCUITS (Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) 8 Claims.

The invention described herein may be manufactured and used by or for the Government of the United States for governmental purposes without the payment to me of any royalty thereon in accordance with the provisions of the act of April 30, 1928 (Ch. 460, 45 Stat. L. 467).

This invention relates to a resonant grounding device for alternating current circuits.

It frequency occurs in power distribution and utilization systems that a power-carrying conductor accidentally makes contact with the frame of a utilizing machine. It is therefore desirable to provide means whereby in such circumstances the fault current will flow through a low impedance path from the machine frame back to the source. Causing the fault current to flow through a low impedance path not only insures that a large enough fault current will flow to operate the safety device which removes the potential from the faulted line, but also prevents the application of high voltage to the machine frame prior to the operation of the safety device.

In direct current distribution systems one of the power conductors may be kept at ground potential and connected to the frames of the utilizing machinery. When Y-connected secondary transformers are used in an alternating current distribution system, an additional conductor may be installed from the neutral point and connected to the frames of machines to be protected. In a delta-connected distribution system there is no neutral unless one is created by an arrangement of specially added transformers, and it is not advantageous to ground one distribution conductor since this causes an abnormally high potential to exist between the other conductors and ground. In some applications, however, such as a distribution system for coal mines, the delta-connected distribution system has certain advantages and is widely used. Safety grounding in coal mines presents a special problem since it is usually difiicult in a coal mine to obtain a low impedance connection to the earth.

One object of this invention is to provide a device to diminish shock hazard in an alternating current system.

A further object of this invention is to provide a device to present a high impedance between distribution power conductors, but a low impedance between the conductors and a fault.

These and other objects will become apparent to one skilled in the art from the following specification, taken in connection with the accompanyingdrawings, in which:

Fig. 1 is a schematic view of a distribution system incorporating the principles of this invention; and

Fig. 2 is a schematic view of the resonant grounding device provided by this invention.

Fig. 3 is a partial schematic view of the resonant grounding device used with a current relay.

Fig. 1 shows a cross section of the earths surface |0 in which there is a coal mine shaft II and gallery l2. bank l4 supplies 3-phase alternating current through power conductors |6, l1 and I8.

Conductors Hi, l1 and l8 descend through shaft into gallery |2 where they are connected to the electrical components of utilizing machines 25-25,, such as a drill or blower. Each machine 25 has a metal frame which is insulated from the electrical component. Each machine 25 is connected to conductors I6, I1 and I8 through safety devices 20, 2| and 22, respectively. Safety devices 20, 2| and 22 may consist of fuses, circuit breakers, or any device operating to open the circuit a predetermined time after an abnormal current flows therethrough. In coal mining it is usual for a number of electrically driven mechanisms to be used in one vicinity, as shown in Fig. l.

Relatively adjacent to machines 2525 there is connected to conductors l6, l1 and I8 a grounding device 28. Grounding device 28, shown in detail in Fig. 2, comprises induction coils 29, 30 and 3|, one end of each coil being connected to a common neutral point 32. Between each pair of the other ends of coils 29, 30 and 3| are connected one of the capacitors 33, 34 and 35. Conductor 36 connects the neutral point 32 with the frames of machines 25. One grounding device 28 will provide a grounding medium for any number of machines.

Coils 29, 30 and 3| and capacitors 33, 34 and 35 are so selected that any two coils, such as 29 and 30, connected in series and this series circuit connected in parallel with capacitor 34 across any two of the power conductors such as l6 and I! will provide a resonant parallel circuit at the frequency of the distribution system. It will thus be seen that a high impedance is provided through the grounding device between any pair of distribution conductors I6, I! and I8. However, the circuit from the frame through the neutral point 32 and any-two of coils 29, 30 and 3| to any two of the distribution conductors l6, l1 and I8 presents a non-resonant and relatively low impedance path for the return of fault cur- A delta-connected transformer.

source: to the frames. of a each machine. .Since the rent to the power system. The inductance coils 29, 30 and 3| of device 28 are preferably wound about a magnetic core, as indicated in Fig. 2, to reduce the size of the device.

In operation, when there is no fault occurring in the distribution system, no appreciable current willflow'through the grounding device because of the high impedance offered by the parallel resonant paths therein. However, if through a breakdown in the insulation of the machine 25, one of the power conductors it, H or H! accidentally comes in contact with a machine frame 26, a low impedance pathwill be provided from the faulted conductor through the frame of motor 25, connecting wire 36, and grounding device 28 back to the distribution system source 14.

Since a low impedance is thus p 'esented to the fault current, the potential of the faulted conductor will immediately drop and, therefore, no high voltage can be applied to the machine frame by reason :of the -fault. .Also, because of the low impedance path provided, a large fault current will flow and the safety devices iii or .22 will quickly operateto remove :the potential from the faulted conductor.

The neutral point 82 of the resonant groundi: devicedil may be grounded bymaking a good connection with the earth, as shown at connection insFig. 2. Alternatively, "the neutral point maybelconnectedto the frames cf the utilizing '30 T machinery, as shownin Fig. .1, w hout such an :earth connection. 'In practice, however, such a device wouldprobably be enclosed aimetalcase, indicated at 93 31111 Fig. 2. which wouldusually be tin contact with the earth. Thereforea pesitive :connection.totheground would :be desirable, and :thiswould normally involve 'connectingthe neutral'point to earth.

Fig. 3 showsa p'art of a modified circuit in which a relay 225i is used to trip cirouit breaker contacts 1 16, ll! andllii,connectingipower cond itors Hi1. -'and lli with the motor .25. Grounding idevicezt is connected .to power conductors M, ii ianddfiias in-Figs. 1 and? Conductor-1 9. (neutralpointz. i ne'cted in series'with thexcperatzng coil of relay :and:the:frarne ofrmotorfii5. Contacts which are closed when relay i is ener breaker which includes cor acts l Tripscoil M andcontacts 'lt connected :acrossone flow of the power'conductors suchas it and I1.

In'Fig. 3. whenia fault occnreand the ;current through the frame of motor and conductor exceeds a predetermined value, relayi Zi will o ateto close contacts i2 43, which are normally open. Operation of relay d0 energizes trip coil i l, causing the opening of circuit breaker contacts 45. 4? and it to remove the ope ating voitage from machine 25. It will h obvoius that a normally closed relay could substituted for nor mally open relay id, in which case contacts 4'2 i3 would be connected in serie'rwith'the low voltage release of .the circuit breaker, including contacts- 56; and '48.

:With the resonant grcu .ding set forth above,theipower'taken from the system rea sonof the; grounding device will negligible due .tothe hi hrimpedance presented by'the par. ll 1 resonant circuit between phases. Because of it will be possible tolocatesuch a device near the working faces in the? mine, thus obviate the installation of :a fourth wire from the power machines tend to be grouped, one grounding device may be provided for each group. Such a grounding device should be materially less expensive than the transformers necessary to establish a separate neutral position on a delta power system, and will have the further advantage of providing a lower impedance to the flow of fault current.

It will be understood that the details of the invention as set forth above are by way of example only, and that many modifications and changes will occur to those skilled in the art within the scope of the appended claims.

What is claimed is:

.1. In a delta-tconnected alternating current distribution system having a plurality of power conductors, a utilizing machine having a metal frame and an electricalcomponent connected to said con 'ductcraa grounding device separate from said ut icing machine havin a neutral point connected to :said frame. said neutral point being connected :throughan inductance I to eachof said conductors, each pairof ends of said inductanccs acent said conductors being connected through capacitor, theinductances and capacitors beso selected that at the frequency of the distrisystem a parallel resonant path is pred between any pairof'conductors.

. -In-a-three-phase delta-connected alternatcurrent distribution system having three 1 power conductors and operating at'a predetermined frequency, a utilizing machine having a :metal and-an electrical-componentconnected to at least two of said conductors, a grounding device'separate from said utilizing-machine having a neutral pointconnected to said frame, said'neutral point beingconnected through an inductance toeach of said conductors, each pair of ends of saidinductances adjacent said conductors being connected'througha capacitor, the incluctances and capacitors being so selected that at the frequency'of the distribution systema parallel resonant path is presented between any pair of power conductors.

3.The combination of claim 2 in which said .neutralipoint islconnected toearth.

i. Thecombination of claim 2 in which there is safety device-connectedbetween said electrical component andsaidaconductors, said safety device :operating to disconnect said electrical component from saidconductors when thecur- .minedifrequency, a utilizing machine having .a metal frame 'andan electrical component :con-

nected to at least two of said conductors, a grounding device having -a neutral point connected to-said frame, said neutral point being connected through an inductance to each ofsaid conductors, each pair of ends of said inductances adjacent said conductors being connected through-a capacitor, the .inductances and capacitorsbeing-so selected that at the frequency of the distribution system a parallel resonant path .ispresentedbetween any pair of power conductors, a. safety device connected between said-electrical component and-saidconductors, said safety device t operating to disconnect said electrical componentfrom saidiconductors when the-cur- .rent. flowing between :said neutral point and said .frameexceeds: a, predetermined value.

6. In a three-phase-delta-connected:alternating current distribution system having three power conductors and operating at a predetermined frequency, a utilizing machine having a metal frame and an electrical component connected to at least two of said conductors, a grounding device having a neutral point connected to said frame, said neutral point being connected through an inductance to each of said conductors, each pair of ends of said inductances adjacent said conductors being connected through a capacitor, the inductances and capacitors being so selected that at the frequency of I the distribution system a parallel resonant path is presented between any pair of power conductors, a safety device connected between said electrical component and said conductors, said safety device including a circuit breaker, said circuit breaker having a trip coil operating to disconnect said electrical component from said conductors when the current flowing between said neutral point and said frame exceeds a predetermined value. v

7. The combination of claim 6 in which the electrical component of the utilizing machine is electrically intsulated from the metal frame thereof.

8. In a three-phase delta-connected alternating current distribution system having three power conductors and operating at a predetermined frequency, a utilizing machine having a 30 metal frame and an electrical component connected to at least two of said conductors, a grounding device separate from said utilizing machine having a neutral point connected to said frame, said neutral point being connected through an inductance to each of said conductors, each pair of ends of said inductances adjacent said conducto-rs being connected through a capacitor, the inductances and capacitors being so selected that at the frequency of the distribution system a parallel resonant path is presented between any pair of power conductors, the electrical component of said utilizing machine being electrically insulated from the metal frame thereof.

LAWRENCE H. HARRISON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,255,387 Chubb Feb. 5, 1918 1,539,834 Gaarg June 2, 1925 1,571,256 Fortescue Feb. 2, 1926 2,122,107 Meller et a1 June 28, 1938 2,180,156 Light Nov. 14, 1939 2,376,201 Starr May 15, 1945 

